1. Field of the Invention
The present invention is directed generally to processes used in the fabrication of solid state devices and, more particularly, to processes used to fabricate gate structures for transistors and the like.
2. Description of the Background
Tungsten silicide (WSi.sub.x) has achieved wide-spread use within the integrated circuit fabrication industry. Specifically, because of its low resistivity, tungsten silicide has been applied as polycide, cladding both the polysilicon gate and interconnect structures. Tungsten silicide is deposited by chemical vapor deposition (CVD) using SiH4 reduction of WF6. That chemistry, however, has a disadvantage because it produces a high level of fluorine. The fluorine diffuses into the gate oxide after a high temperature anneal and degrades the gate oxide strength.
The migration of fluorine into the gate oxide to degrade the gate oxide has been known for a number of years. A paper entitled "Direct Evidence of Gate Oxide Thickness Increase In Tungsten Polycide Processes" by S. L. Hsu et al. IEEE Electron Device Letters, Volume 12, No. 11, November 1991, pp. 623-625, demonstrates that fluorine atoms migrate from a tungsten silicide layer to a gate oxide layer causing additional oxide growth. The additional gate oxide causes device degradation such as causing shifts in threshold voltages and decreases in saturation current.
One attempt at solving the problem is found in U.S. Pat. No. 5,364,803 entitled "Method of Preventing Fluorine-Induced Gate Oxide Degradation in WSI.sub.x Polycide Structure". Disclosed in that patent is a method wherein a thin, conducting, diffusion barrier layer is deposited over the gate polysilicon layer. The thin, conducting, barrier layer is preferably sputtered titanium nitride (TiN.sub.x) having a preferred thickness of between about 150 to 1,500 angstroms. N.sub.2 is fed into a sputter chamber with a titanium target. The sputtered titanium atoms react with the N.sub.2 to form TiN.sub.x on a silicon wafer. Other conductive barrier layers for the diffusion barrier layer are titanium tungsten (TiW) or tantalum nitride (TAN) which are sputtered in a way similar to the sputtering of the titanium nitride.
The barrier layer disclosed in U.S. Pat. No. 5,364,803 minimizes the number of fluorine atoms diffusing from the tungsten silicide layer into the gate oxide layer. However, the barrier layer thus produced may increase the resistance of the gate structure. Also, the sputtering process may be inefficient and difficult to control such that the thickness and properties of the barrier layer cannot be precisely controlled. Furthermore, the sputtering process adds material to the gate structure, thereby increasing the height of that structure. Thus, the need exists for a method of constructing a barrier layer having characteristics which can be precisely controlled using commercially available process steps. The need also exists for a barrier layer of low resistance and which does not add height to the gate structure.